so glad i've never really had trouble sleeping.. if anything i probably get too much sleep XD
still have the nightowl habits though, so usually have to force myself to bed when i need to get up early
but no trouble at all with the actual falling asleep bit even when i force myself to bed ^^

I stayed up past midnight routinely in my college years, but now, I'm pretty tired by about 22h00. Lack of sleep can be a pretty big problem. In medical school and residency, I've heard stories of residents falling asleep while interviewing patients or performing procedures. Myself, I can recall trying to study in the library after a night on call, and reading the same paragraph five times or more and yet immediately afterwards not remembering a word of what I had just read.